Best language to learn?

Few of us are logical, so neither are most human languages. Esperanto, being an invented language, is the most logical. Most West European languages in their formal versions became more logical during the "Age of Reason" aka "The Enlightenment" of the late 17th century and the 18th century. This applies particularly to French where "I drink coffee from a cup" becomes "Je bois de café dans une tasse" because the coffee in within the cup when you start drinking it. And whereas Portuguese would say "Tinha dois mas não acho" and English "I did have a couple but now I cannot find either of them", French insists on "J'en avais deux mais alors je ne puis pas les trouver, ni l'un, ni l'autre." German also scores high in being very precise: "Aufwiedersehen" (till we see each other again) becomes "Aufwiederhören" (till we hear each other again) on the telephone, and William Joyce who broadcast in English for Nazi Germany was once forced to report that a seaplane had watered on the North Sea "because even English could not be so illogical as to refer to a plane landing on the sea!" However it was the Romans who anathemetized the traditional double negative, with an excess of logic that got their prohibition imported into "correct" English by 16th century schoolmasters.
But why regard an excess of rationality as an endorsement of quality in a language. Humans use languages for so much more than mere conveyance of arid intellectual information: the "best" language is one that is also good to express enjoyment or anger, to joke in, to make love (or war) in, to express sympathy, grief, and a myriad of relationships and emotions, and with a wide range from stern formality to flippant colloquialism. For my money that would be Brazilian Portuguese (and it is why I so much prefer it to, e.g. Spanish), but I concede that other people may have different preferences.

Either a world language that's spoken across great parts of the world, like for example Spanish or French, or a hard language that's very much in demand, like for example Arabic or Mandarin Chinese.

Not an easy language to learn but I recomend French. It is the official language of the United Nations, it spoken over wide swaths of Africa, the Caribbean, Polynesia, North America and throughout Europe. If you are job hunting on the International market then there is no better language to have on your resume than French. That is besides English of course.

Logical language? If you mean logical with respect to regularity, then I would suggest: Hungarian, Finnish, Turkish, Basque, or Quechua.

If by logical you mean a language that is most spoken and will be beneficial in business: Spanish, French, German, Mandarin, Japanese, Italian, Portuguese, Russian, Arabic
beneficial for govt jobs and translating: Farsi, Arabic, Indonesian, Turkish, Pashto, Azerbaijani, Russian, Hindi, Bengali, Urdu

From the first group, I would choose either Basque or Hungarian.
second group, I would choose French or German
third group, I would choose Indonesian

When you say "logical language" do you mean a language that has very few irregularities, thus is easy to learn, or do you mean a language that is widely used across the world, thus would pay back the effort of learning it?

Learn to use your own language logically before.....!

If you're in the US, I'd say Spanish; it's one of the most common languages besides English.
Then again, Esperanto would easily be the best because it was invented and meant to be spoken in lieu of a native tongue.

ASL aka sign language is much easier. Then again not much is spoken/signed. If you are in the states and don't really plan traveling maybe try so spanish/espanol. Messing with French, Italian and Espanol I notice a lot of similarities. Keep in mind Catalan and Portuguese are Spanish but a little different.

If you live in the US, I recommend learning German, speakers of this language make much more than French or Spanish.

If you mean the most useful, clearly English. If you speak English already, the most useless choice would be French---every French speaker of consequence already speaks English far better than you will ever speak French. If you already speak English, I dunno. If you didn't start as a child, you will never speak Chinese (Mandarin or Cantonese) or Japanese well enough to do business without a translator. Spanish is the next most widely spoken language, but unless you have a specific need for it, most of them also speak English. Personally, I would skip it and double up on science, technology, engineering and math.

Sign Language